Corrosion inhibiting method

ABSTRACT

A LIQUID CORROSION INHIBITOR AND METHOD DESIGNED FOR USE BY WATER TREATMENT PLANTS FOR THE CONTROL OF CORROSION OF MUNICIPAL, INDUSTRIAL, COMMERCIAL AND DOMESTIC METAL PIPING SYSTEMS, WHEREIN AS THE INHIBITOR IS ECONOMICALLY ADDED TO THE FLOWING WATER SUPPLY IN MINUTE DOSAGES, IT PASSIVATES THE SURFACE OF THE METAL PIPING AS IT DEPOSITS A SELF-CONTROLLED FILM OF INSOLUBLE ZINC PHOSPHATE. ONE OF THE PREFERRED INHIBITORS INCLUDES ZINC, SULFAMATE, AND ORTHOPHOSPHATE IONS. TO BE EFFECTIVE, THE PH MUST BE CONTROLLED TO WITHIN THE RANGE OF 5-9.

United Statesv Patent ice 3,669,615 Patented June 13, 1972 3,669,615CORROSION INHIBITING METHOD William Bruce Murray, 929 Terraine Ave.,Long Beach, Calif. 90804 No Drawing. Continuation-impart of applicationSer. No.

829,842, May 5, 1969. This application Sept. 28, 1970,

Ser. No. 76,221

Int. Cl. C23f .7/

US. Cl. 21-2.7 10 Claims ABSTRACT OF THE DISCLOSURE CROSS-REFERENCES TORELATED APPLICATIONS This is a continuation-impart of Ser. No. 829,842,filed May 5, 1969 by William Bruce Murray, entitled Method of InhibitingCorrosion of a Ferrous Surface, now abandoned.

BACKGROUND OF THE INVENTION Iron, when immersed in a supply of aeratedwater which is slightly acid to slightly alkaline (pH 6.5-8.3), developslocal anode and cathode sites on its surface. These sites or cells,formed as a result of impurities or intermolecular stresses in the iron,have a potential of 0.41 volt, which sponsors a current flow through thebase metal and the electrolyte (water supply). At the anode iron isconverted from metallic to ionic state, while at the cathode eitherionic hydrogen or dissolved oxygen is reduced, causing the surface ofthe cathode to become alkaline, reference the followin formulae:

(A) (Anode reaction) Fe=Fe+++2e (B) (Cathode reactions) 1) Oxygenreduction 2e+H O+ /z0 =2OH- (2) Ion reduction 2e+2H O=2OH-+H (C)(Combining of the two electrode products yields ferrous hydroxide):

Fe+++2OH-=Fe(OH) FIELD OF THE INVENTION The process and composition areeach adapted to the treatment of open and closed water systems such asmay contain potable or non-potable flows. These objects are adaptable toprotection of ferrous and non-ferrous metals against corrosive waters,including variations between aerated soft meteorological and high totaldissolved solids water supplies.

DESCRIPTION OF THE PRIOR ART The known prior art is best represented byPat. No. 3,284,368 including a zinc and hexametaphosphate corrosioninhibitor. Calcium carbonate as an inhibitor is also currentlypracticed.

Other non-anticipatory art includes the following patents andpublications:

2,080,299 Benning et al. 2,316,810 Romig. 2,711,391 Kahler. 2,793,932Do.

3,024,201 Bergman. 3,116,178 Upham. 3,120,425 Hatch. 3,151,087 Ryznar etal. 3,347,797 Kuegemann et al.

Butler, Ison; Corrosion and Its Prevention in Waters,

Reinhold, 1966. Chap. 1, p. 2.

Uhlig; Corrosion and Corrosion Control, Wiley, 1965.

Chap. 6, p. 79.

Laque, Copson; Corrosion Resistance of Metals and Alloys, Reinhold, 2nded., chap. 4, p. 83.

SUMMARY OF THE INVENTION A process and composition of matter efl ectivein minimizing pitting and tuberculation on the interior surfaces offerrous and non-ferrous water distribution plumbing systems. Corrosivewaters ranging from aerated soft meteorological to high total dissolvedsolid supplies are initially treated herein through the introduction ofsmall but elfective quantities of "zinqsulfamate-orthophosphateinhibitor, whereby the inhibitor solution, when applied to the Water,passivates the surface of the corroding interior metal of piping systemsby depositing a zinc phosphate film thereover, effectively preventingdiffusion of dissolved oxygen. Selective diminution in application a ofthe inhibitor solution to 1 p.p.m. zinc, or less, in the corrosive watersustains the protective zinc phosphate film.

As either a potable or non-potable water addition, the zinc, sulfamicand phosphate ions would not exceed 3 p.p.m. The pH of the treated wateris to be maintained at between 5 and 9; preferably for potable water atbetween 6.5 and 8.4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS To explain the theory ofpassivation hereunder, consider the inhibitor at a dosage of 3 p.p.m.zinc or less is being applied to a slightly acidic to slightly alkalinewater (pH 6.5-8.3) Of the ions contained in the inhibitor, zinc, sodium,hydrogen, phosphate, sulfate and sulfamate, the zinc and phosphate ionstend to precipitate in the water environment but are usually in thecolloidal state.

This compound is preferably so finely divided that it resists settlingfrom the bulk of the electrolyte, and it can only be removed bycentrifuging and ultrafiltration. The zinc phosphate has a veryimportant characteristic in that it adheres tenaciously to most of thecommon plumbing material s, i.e. metals, glass, plastic or rubber. Filmstudies on glass and metals have indicated a deposit in the order of amolecule thickness (1 10'- in.) at zinc dosage rates of 1 p.p.m. zincadded continuously to a flowing alkaline (pH 8.3) water supply. If theinhibitor is not applied at dosage values exceeding 2 to 3 p.p.m. zinc,the film will not deposit on itself and accumulate as in the case of athickly precipitated calcium carbonate scale.

Metals are protected by the present thinly deposited zinc phosphatefilm, which presumably is insoluble in water, and as such it restrictsthe corrosive effects of the gases, dissolved oxygen and carbon dioxide.This film attaches itself so firmly to materials that it cannot beeasily washed or rubbed off. It is, of course, subject to the abrasiveaction of suspended solids in a flowing water stream and in theenvironment of a mineral or a strong fruit acid it dissolves. Thus, oncea freshly installed metal is exposed to water treated with theinhibitor, it can be anticipated that the surface of the metal will bepassivated and the process of normal metallic corrosion will beprevented. Existing pre-rusted or corroded metals will be affordedprotection in such a treated water supply because of the passivationinduced by the film deposited on the surfaces of the tubercules or otherdeposits. Once a plumbing system has become passivated, as is evidencedby the presence of the zinc phosphate at the extremities of thedistribution system or termination of the metallic plumbing, the initialpre-coat dosage of inhibitor may be dropped to some low value so as tojust maintain the deposited film and to protect against mechanicaldamage.

As indicated, the formulation develops the zinc, sulfamate and phosphateions, through either a combination of zinc oxide, sulfamic acid andphosphoric acid or zinc sulfate, sulfamic acid and monosodiumorthophosphate. Suitable sources for the zinc include zinc chloride,zinc oxide, zinc sulfate, metallic zinc, zinc carbonate, sodium zincateand/or zinc phosphate. Suitable sources for the phosphate ion includeorthophosphoric acid or salts of orthophosphoric acid. Sources of thesulfamate ion include principally sulfamic acid. This ion is importantin part in retaining the zinc in solution against minor pH fluctuations.

The mol ratios are maintained, regardless of the starting product andare generally represented by the formula comprising 3 atoms of zinc to 2mols of phosphate, thus:

Zn(PO4)2 ZNaNHzSOa 3HZSO4 Ten gallons of an aqueous solution containingone pound of zinc per gallon can be prepared by adding a fifty poundcomposite of the ingredients to 8.25 gallons of water:

Lbs.

ZHSO4'H2O HNH SO 10.00 NaH PO 50.00

The inhibitor solution has the following physical characteristics:

Color Water white. Baum 42.9 at 20 C. pH (1.0 at 20 C.

Ten gallons of the above solution applied to one million gallons ofwater will supply zinc at a level of 1.2 parts per million.

As mentioned above, dosage of the inhibitor to a corrosive water shouldpreferably be maintained initially at a zinc concentration of 2 to 3parts per million. This dosage should be maintained for a length of timeas dictated by the level of zinc sampled at some remote point in thedistribution system. When it has been determined that the zinc residualhas approached the applied dosage, the treatment rate should be reducedto a continuous zinc dosage of one part per million or less.

The liquid and dry inhibitor formulations for potable water are alsosatisfactory for non-potable water treatment. Some acid and/ormetaphosphate addition might be necessary for non-potable Waters becauseof the problems associated with water concentrating (high totaldissolved solids). An example of this application would be forindustrial, non-potable water re-circulating and single pass supplysystems containing large quantities of salts, 1.e., sea water.

It has been found in practice that the orthophosphate is most desirablein contrast to the common metaphosphates and polyphosphates. Theorthophosphate ion competes more favorably for the metal in waters highin chlorides and sulphates which normally tend to dissolve anode andcathode ru st deposits. Zinc orthophosphate per se, being insoluble isnot affected by water temperature fluctuation and has the essentialeffect of developing an oxygen impenetrable barrier at the corrosionsite.

Tables I and II demonstrate turbidity variations due to the effect of pHand the presence of metaphosphate residual on highly buffered water(Colorado River water) treated with various dosages of inhibitor at one,two and three p.p.m. zinc. Of particular interest is the presence ofmetaphosphate which prevented the interference of calcium carbonateprecipitation due to the so-called threshold effect. The cold watertemperature Was 24 C. While the hot water temperature of the samples wasadjusted to C. for three hours.

TABLE I Efiect of pf! on turbidity of Colorado River water at varioustreatment levels of zinc sulfate-monosodium phosphate-sulfamic acidinhibitor Turbidity in Jackson Turbidity Units 1 Cold 0-) Hot (80 0.):

1 p.p.m. 2 p.p.m. 3 p.p.m. 1 .p.m. 2 .p.m. 3 .m. Zinc Zine Zinc p11 pZine p Zinc p imo 1 Turbidity measurements made with a Each LaboratoryTurbidimeter are in Jackson Turbidity Units.

2 Turbidity read cold (24 C.) and hot (80 C.). The 80 0. temperature wasmaintained for 3 hours before reading.

3 The pH adjustments were made with 0.02 N H2804 using 9. FisherTitralyzer.

TABLE II Effect of sodium hexametaphosphate on turbidity of ColoradoRiver water at various treatment levels of zinc sulfate-monosodiumphosphate sulfamic acid inhibitor Turbidity in Jackson Turbidity Units 1Cold (24 C.) Hot (80 C.)

1 p.p.m. 2 p.p.m. 3 p.p.m. p.p.m. 1 p.p.m. 2 p.p.m. 3 p.p.m. Zinc ZineZinc SHMP Zinc Zine Zine 1 Turbidity measurements made with a EachLaboratory Turbidimeter are in "Jackson Turbidity Units.

8 Turbidity read cold (24 C.) and hot (80 0.). The 80 0. temperature wasmaintained for 3 hours before reading.

3 Sodium hexametaphosphate.

Norm-11H of Water at start of test was 8.4.

A specific example of the performance of the corrosion inhibitor hereinis as follows:

SAE 1010 carbon steel coupons were exposed to 0.5 gallon per minutestream of water for thirty days. The corrosion rate of the controlspecimen, untreated, was 85.7 m.d.d. (milligrams per square decimeterper day), while the inhibitor treated specimen showed a corrosion rateof less than 1.0 m.d.d. Inhibitor film analysis showed that the treatedspecimen had been zinc phosphatized. The inhibitor formulation was addedto the water stream on a continuous basis resulting in a treatment levelequivalent to 1 p.p.m. zinc.

I claim:

1. In the conduct of water by means of plumbing systems, a method ofinhibiting the corrosion of ferrous conductors thereof, comprising thesteps of:

(A) initially treating the water with a small but effective dosage of aninhibitor including zinc, sulfamate, and orthophosphate ions;

(B) simultaneously controlling the pH of the water within the range of5-9; and

(C) sequentially passing the treated water into continuous contact withthe conductors to passivate and coat corroded conductive surfacesthereof by depositing a protective film thereon.

2. The method according to claim 1, wherein as the precoating iseffected and zinc residual approaches applied dosage, a sequential andcontinuous reduction in the dosage is efr'ected, said reduction indosage not exceeding the required amount to sustain the deposition ofzinc phosphate film.

3. The method according to claim 1 wherein zinc ions are derived fromzinc chloride.

4. The method according to claim 1 wherein zinc ions are derived fromzinc sulfate.

5. The method according to claim 1 wherein zinc ions are derived fromzinc phosphate.

'6. The method according to claim 1 in which phosphate ions comprisingthe deposition film of zinc phosphate are derived from orthophosphoricacid.

7. The method according to claim 1 in which phosphate ions comprisingthe deposition film of zinc phosphate are derived from salts oforthophosphoric acid.

8. The method according to claim 1 in which sulfamate ion is derivedfrom sulfamic acid.

9. The method according to claim 1 including the additional step ofadding in the initial dosage 0.53.0 p.p.m. sodium hexametaphosphate toprevent precipitation of calcium carbonate.

10. The method according to claim 9 wherein as the pre-coating iseffected and zinc residual approaches applied dosage, a sequential andcontinuous reduction in the dosage is effected, said reduction in dosagenot exceeding the required amount to sustain the deposition of zincphosphate film.

References Cited UNITED STATES PATENTS 2,316,810 4/1943 Romig 212.7 X2,332,209 8/ 1943 Enquist 2l-2.7 X 2,742,369 4/ 1956 Hatch 106-142,877,085 3/1959 George 21-2.7 2,900,222 8/1959 Kahler 212.7 3,116,10512/1963 Kerst 212.7 3,151,087 9/1964 Ryznar 2l2.7 X 3,510,436 5/1970Silverstein et al 252-389 OTHER REFERENCES Betz Handbook of IndustrialCorrosion, 6th ed. (1962) pp. 265, 266.

MORRIS O. WOLK, Primary Examiner D. G. MILLMAN, Assistant Examiner US.Cl. X.R.

